1. Field of the Invention
The present invention relates to an assembled battery total voltage detection circuit, in particular, an assembled battery total voltage detection circuit that measures voltage divided by a divider resistor connected between a plus (+) terminal and a minus (−) terminal.
2. Description of Related Art
A total voltage detection circuit with insulated input and output is conventionally used in, for example, an assembled battery for a pure electric vehicle or a hybrid electric vehicle. The insulation is required because a vehicle occupant otherwise may get a shock when high voltage shorts to a chassis ground of a vehicle. Therefore, in a pure electric vehicle or a hybrid electric vehicle, a main control circuit connected to the chassis ground of the vehicle and a circuit for a high voltage system are insulated from each other by high resistance.
FIG. 2 shows an example of the total voltage detection circuit for an assembled battery. In the circuit in FIG. 2, a total voltage of an assembled battery 1 is divided to a low voltage by using resistors 2 and 3 so that the low voltage constitutes an input voltage into an A/D converter 4. The A/D converter 4 operates with an insulation power source 6 utilizing a reference voltage source 5 as a reference voltage. The A/D converter 4 is insulated via a photo-coupler portion 7. A microcomputer 8 performs operation control and data communication with a power source insulated from a high voltage system.
For an assembled battery total voltage detection circuit, Japanese Laid Open Patent Publication No. 2004-219414 discloses a technique in which a divider resistor constituted by a series circuit including a high resistor R1, a resistor Rm1, a resistor Rm2, and a high resistor R2 is connected between a plus terminal and a minus terminal of an assembled battery, and a voltage divided by the divider resistor is measured via a differential amplifier circuit, with a connection point (midpoint of the divider resistor) of the resistors Rm1 and Rm2 connected to a ground. Japanese Laid Open Patent Publication No. 2001-124805 discloses a technique for measurement through converting an assembled battery voltage to a current via a resistor and performing voltage-to-current conversion while insulating.
However, the conventional assembled battery total voltage detection circuit shown in FIG. 2 has a problem that an A/D converter is required besides a main control circuit which is constituted by an insulation power source, a microcomputer, and the like, thereby resulting in an increase in production costs. In the technique disclosed in the patent publications quoted above, high resistance is required to increase voltage divider ratio so as to divide voltage into one that an A/D converter can measure. In this case, current that flows through a total voltage detection terminal (connector) is reduced. Therefore, a costly gold-plated terminal is required so as to secure reliability of contact point of the total voltage detection terminal.
In view of these circumstances, the present invention intends to provide an assembled battery total voltage detection circuit, assuring high reliability and reduction in cost.